Chronic pain originating from the musculoskeletal system is a dominant cause of sick-leave in modern industry and can be a very disabling and troublesome condition for the individual. Although the cause of this problem in skeletal muscle is unknown, one of the most frequent situations in which muscle pain is experienced is in industrial workers who have to move repeatedly and/or forcibly. The cumulative trauma disorder (CTD) which results from repetitive movements is of special interest because these repeat-motion injuries are one of the most difficult to anticipate and prevent. Our studies in humans have shown that exposure to a single bout of repeated strains can lead to myofiber and fascial rupture without bleeding but accompanied by muscle pain, restricted motion, and loss of strength and power. Little is known about the effect of repeated strains on muscles or the dynamic components of repeated use such as velocity and acceleration which produce injury resulting in CTD or CTD risk. Since variations in human exposure and response together with the necessity for repeated tissue sampling makes man unsuitable as a research subject, we have developed a rodent model of repeated strain injury (CTD). Using this model, the present study is designed: 1) to determine the dynamic factors (velocity, acceleration and dose) which produce dysfunctional versus adaptive muscles, 2) to document changes in the extracellular matrix and myofibers which lead to a pathologic muscle, and 3) to study the functional outcome and reversibility of repeated injury at different speeds and accelerations commonly experienced by hand- intensive industrial jobs. This research consists of experiments in which muscles are chronically injured by mechanical overloading in deeply anesthetized rats. The tissues are surveyed at various time intervals by biochemical, immunohistochemical and histological techniques for specific cellular markers, components and mediators involved in tissue injury and repair. The functional outcome of repeated injury is assessed by in vivo dynamometry; strength, endurance and stiffness are good measures of muscle performance. Insight into the dynamic factors producing muscle injury should provide a better understanding of the healing (adaptive) or failed-healing (pathologic) processes of muscle and aid in the design of preventative regimens for individuals in specific industrial settings. The long range goals are to determine: 1) if diminished muscle shock absorption is an impOrtant step in the development of clinical CTD; 2) if prevention of CTD can be implemented by behavioral alterations.